The invention relates to thermosensitive flowmeters that detect the flow rate of a fluid using a thermosensitive resistor.
Flowmeters that detect the flow rate of a fluid to be measured using a balanced bridge circuit having a thermosensitive resistor arranged in the fluid are disclosed in, e.g., Japanese Utility Model Unexamined Publication No. 108930/1986 and Japanese Patent Unexamined Publication No. 216214/1989.
FIG. 2 shows the configuration of a thermosensitive flowmeter. A detecting tube 2 is disposed at a predetermined position in a housing 1 that serves as a main path of a fluid. A thermosensitive resistor 3 and a fluid thermometer 4 are disposed at predetermined positions in the detecting tube 2, constituting a bridge circuit together with resistors R.sub.1, R.sub.2. The input terminals of a differential amplifier 101 are connected to nodes b, f of the bridge circuit, whereas the output terminal of the differential amplifier 101 is connected to the base of a transistor 102. The emitter of the transistor 102 is connected to one terminal a of the bridge circuit, and the collector of the transistor 102 is connected to a power supply 103.
FIG. 3 shows the structure supporting the thermosensitive resistor 3. The thermosensitive resistor 3 having a heating section (temperature-dependent resistor film) 31 on a surface of a plate-like insulating substrate 30 is attached to the detecting tube 2 through a supporting member 5 that supports an end of the thermosensitive resistor 3. The thermosensitive resistor 3 is connected to terminals 6 through power supplying lead wires 34, the terminals being supported by the supporting member 5 by insertion. FIG. 4 shows the detailed configuration of the thermosensitive resistor 3, in which reference numeral 31 designates the heating section (temperature-dependent resistor film); 32, an electrode section; and 33, a support section.
Since the operation of the thus configured conventional thermosensitive flowmeter is known, a detailed description thereof will be omitted. This circuit becomes balanced when the voltage at node b equals the voltage at node f. At this point in time, a current I.sub.H corresponding to the flow rate flows through the thermosensitive resistor 3. The voltage V.sub.0 at node b becomes I.sub.H x R.sub.1, and is used as a flow rate signal.
However, the voltage V.sub.0, which is a flow rate signal at the time the voltages at nodes b, f are equal to each other, has errors. The errors include: dimensional and profile errors of various parts, errors of constants of the respective resistors forming the bridge circuit, and temperature distribution variations at the heating section 31 of the thermosensitive resistor 3. While it is common to give a trimming area N to the thermosensitive resistor 3 to keep the resistance thereof constant, the trimming area N is sometimes trimmed to such a profile as to be coincident with a nontrimming area M or not trimmed at all due to variations in the material or film thickness of the resistor.
If the trimming area N is trimmed so as to be coincident in profile with the nontrimming area M, a substantially uniform temperature distribution can be obtained during conduction of the heating section 31. If the trimming area N is not trimmed at all, it is apparent that a low temperature distribution is observed at the trimming area N of the heating section 31.
FIGS. 5 (a) and (b) show temperature distributions of the thermosensitive resistor 3. FIG. 5 (a) shows a case where all the trimming area N has been trimmed whereas FIG. 5 (b) shows a case where no trimming area N has been trimmed. Reference characters h.sub.0 to h.sub.9 indicate positions along the length of the thermosensitive resistor 3, corresponding to h.sub.0 to h.sub.9 in FIG. 4. For flow rate adjustment, a variable resistor R.sub.2 is adjusted so that the flow rate signal voltage V.sub.0 takes a predetermined value when the fluid is applied at a constant flow rate. However, when this adjustment is made on heating sections whose trimming amounts are different such as the cases (a) and (b), not only the temperature distributions but also average temperatures T become different. Therefore, a predetermined output can be obtained at an adjusted flow rate, but errors result at flow rates other than the adjusted flow rate since the flow rate characteristic of the flow rate signal voltage V.sub.0 changes. In addition, the temperature characteristic also contributes to aggravating such errors.